This invention relates to improvements in tractor-trailer pneumatic brake systems.
An important component of our national transportation system is the highway tractor. A substantial portion of manufactured goods and raw materials are transported over the nation's roads and highways in trailers which are hauled by highway tractors. A single highway tractor is capable of transporting a trailer having a load which can run into the order of many tons.
Pneumatically-operated brake systems have been adopted for such tractor-trailer combinations. Likewise, the "fifth wheel" has been adopted as the mechanical means by which a trailer connects to a tractor.
In a tractor-trailer combination, the trailer brakes must be placed under the control of the driver in the tractor. This is accomplished by appropriate pneumatic circuitry on the tractor which is connected via air lines with compatible pneumatic circuitry on the trailer when the trailer is mechanically connected to the tractor's fifth wheel for hauling. The pneumatic circuitry on the tractor contains various valves through which control over the tractor's and trailer's pneumatic brakes is exercised by the driver.
The tractor contains a pneumatic power supply which comprises an air compressor driven by the tractor's engine to draw and compress air into one or more tanks which form a reservoir or reservoirs of compressed air at suitable pressure for operating the pneumatic brakes of the tractor and those of a trailer when such a trailer is connected to the tractor for over-the-road hauling. The compressor is selectively loaded and unloaded in accordance with pressure conditions in the tank to maintain a certain pressure of compressed air in the tank or tanks. Such tanks are typically mounted on the chassis of the tractor, and pneumatic lines extend from such tanks to pneumatic valves located in the tractor cab for use by the driver in exercising control over the tractor's brakes and those of a trailer when connected to the tractor for hauling.
The service brake system of the tractor comprises a foot pedal operated pneumatic valve which the driver uses to apply the service brakes of the tractor's wheels and also those of a trailer when connected to the tractor. The tractor cab also contains a pneumatic trailer brake valve which the driver can operate by hand, enabling the driver to apply the trailer service brakes in a controlled manner independently of the tractor's service brakes. This trailer brake valve gives the driver certain control which can be important at certain times in the operation of the tractor-trailer combination.
Additionally, the tractor-trailer combination comprises parking brakes commonly of the type known as spring brakes. The spring brakes are not a substitute for the service brakes; rather they are intended to lock the wheels when the tractor-trailer combination is parked. When the tractor is disconnected from the trailer, the trailer's parking brakes are automatically applied to prevent trailer roll-away. When no trailer is connected to the tractor, the tractor parking brakes may also be applied by the driver to look the tractor wheels. Controls for operating the parking brake systems are in the tractor cab for manual activation by the driver. The parking brakes of both tractor and trailer also are adapted for operation in response to certain conditions of the service brake system, such as when the pressure of compressed air in the tractor's compressed air storage tank or tanks drops below a certain threshold.
In the fully pneumatic air brake systems which are currently in use, pneumatic lines are routed into the tractor cab where the pneumatic brake controls are located. For convenience of the driver of the tractor, the tractor and the trailer parking brake controls are typically mounted on the dash, and it is therefore necessary to route pneumatic lines to this area of the cab.
One aspect of the present invention relates to a new and unique organization and arrangement of various component parts of a pneumatic-operated brake system of a tractor, both for the tractor per se and for the tractor-trailer combination. This novel organization and arrangement can contribute to reducing manufacturing and maintenance costs. Certain of the pneumatic components heretofore mounted inside the cab are disposed external to the cab on the tractor chassis. Control of the pneumatic brakes is accomplished through an electro-pneumatic system rather than a system which is strictly pneumatic. More compact electrical and electronic controls are mounted in the cab's dash and connected by electric wires to pneumatic hardware components which are mounted on the chassis external to the cab. This frees the space in the area of the dash and facilitates installation of the controls in the cab. The use of electric wires instead of air lines between the cab and the chassis also provides a simplification. The number of air line couplings is reduced, a significant cost saving, and the installation of electric wires is considerably easier than installing pneumatic air lines. Attributes of the invention are especially significant in cab-over type tractors where lines between the cab and the chassis must be routed through the pivot point of the cab on the chassis.
A still further advantage of the invention is that changes which may be required, such as by governmental regulation, can often be accomplished more efficiently and economically because it is often possible to make changes in electronics instead of in hardware, such as valves, lines, and couplings.
Further consequences of the improvements afforded by the invention are reduction in cab noise levels due to removal of certain pneumatic components from the cab, and a modest, but finite, reduction in vehicle curb weight.
Another aspect of the invention relates to the manner in which the air compressor is operated and the air dryer is purged of condensed moisture. The compressor is driven by the tractor's engine and it is loaded and unloaded in accordance with the level of pressurized air in the storage tank or tanks. Because of the inherent nature of the air compression process, moisture condenses in the compressed air and must be removed. Existing compressors include governors which control compressor loading and unloading in accordance with the level of air pressure sensed at the storage tank or tanks. A pneumatic line transmits the tank pressure to the compressor governor, and when the sensed pressure drops below a threshold, the compressor is loaded to the tank, drawing air and compressing it into the tank. When the tank pressure rises to a certain level, the compressor is once again unloaded, ceasing to compress air into the tank. This cycle periodically repeats in accordance with demand, serving to maintain tank pressure within a certain range.
In the previous design, purging of moisture from the pneumatic system is concurrent with unloaded operation of the compressor. Because a substantial proportion of the tractor's operating time comprises the compressor being unloaded, the dryer purge time is much greater than necessary to accomplish sufficient purging. During purging of the dryer, a low pressure path is created between the compressor intake port and atmosphere. In a tractor which has a turbo-charged engine the compressor may draw the boost air from the turbo-charger. Consequently, when purging occurs, boost pressure may be lost through this low pressure path resulting in some attenuation of engine power; this may be manifested at times by the need for the driver to downshift in order to overcome such temporary power attenuations. The mechanical governor system for the air compressor may also require periodic maintenance which often includes the need for re-calibration.
A still further aspect of the invention relates to improvements in the compressor loading-unloading cycle and dryer purge cycle which can produce greater accuracies and consistencies than in mechanical-pneumatic systems, thereby making maintenance and re-calibration less frequent. Certain adjustments, when needed, can be accomplished in the electronics, rather than in hardware. The improvements are compatible in many instances with compressors and dryers currently in use so that the improvements can be incorporated without the necessity of completely re-tooling the system.
A still further aspect of the invention relates to improvements in the parking brake control system. As alluded to earlier, current systems use rather large, complicated pneumatic valves located in the dash area of the tractor cab. Requirements for sequencing and low-pressure cut-out have added to the complexity, cost, and size of components that must be placed in the dash, and there are increased interior noise levels due to the routing of air lines into t he cab. According to this further aspect of the invention, certain pneumatic circuit components are replaced by novel electro-mechanical control devices, and associated electronics, in the cab. These particular control devices are the parking brake controls for the tractor and the trailer. There are two such devices. One is for the trailer alone; the other is for the tractor, but it also interacts with a trailer when connected to the tractor such that the trailer parking brakes are operated if the driver operates the tractor parking brakes. The sequencing function of applying the trailer parking brakes just prior to the tractor's in embodied in the electronics. The devices are also interactive via the electronics with an electric pressure transducer which senses the level of air pressure in the tank or tanks such that if the sensed pressure drops below a certain threshold, the parking brakes are automatically applied. The electronics however also contains an alarm to alert the driver to this possibility before it occurs so that typically there will be time for the drive to initiate action himself before the parking brakes are applied. The pressure transducer signal is also used by the electronics in controlling the compressor loading-unloading and dryer purge functions.
The foregoing features, advantages, and benefits of the invention along with additional ones, will be seen in the ensuing description and claims which should be considered in conjunction with the accompanying drawings. The drawings disclose a preferred embodiment of the invention according to the best mode contemplated at the present time in carrying out the invention.